1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device, and particularly to a nonvolatile semiconductor memory device performing data writing in a toggle manner.
2. Description of the Background Art
An MRAM (thin film Magnetic Random Access Memory) is a general name of solid state memories that utilize a magnetization direction of a ferromagnet for data storage. In the MRAM, the magnetization direction of the ferromagnetic substance forming a memory cell is set parallel and anti-parallel to a certain reference direction in accordance with data “1” and “0”, respectively. A GMR (Giant Magneto-Resistance) element utilizing a GMR effect for reading data of the memory cell, a TMR (Tunneling Magneto-Resistance) element utilizing a tunneling magneto-resistance effect for reading the data and the like are used as the MRAMs.
The TMR element is formed of three layer films of ferromagnetic, insulating and ferromagnetic layers, and a tunneling current flows through the insulating layer. A resistance value with respect to this tunneling current changes in accordance with a relationship in magnetization direction between the ferromagnetic layers.
An external magnetization inverting method has been known as a method of inverting the magnetization direction of the ferromagnetic layer. In this external magnetization inverting method, an external magnetic field is generated by passing a current through the vicinity of the memory cells, and thereby the magnetization direction of the ferromagnetic layer is inverted. In this external magnetization inverting method, however, the external magnetic field affects magnetization of a memory cell other than a write-target memory cell, and particularly the memory cell that is located one at least one of a bit line and a digit line corresponding to the write-target memory cells (and may also be referred to as a “half-selected memory cell” hereinafter). Thereby, the external magnetic field may cause a malfunction.
For overcoming the above problem, a toggle manner has been known as a manner of writing data into the memory cell. In the toggle manner, the TMR element is formed of a fixed magnetic layer that is a ferromagnetic layer having a fixed magnetization direction, a free magnetic layer that is a ferromagnetic layer having a variable magnetization direction and an insulating layer. The free magnetic layer in the toggle manner has an SAF (Synthetic Anti-Ferromagnetic coupling) structure. Thus, the free magnetic layer includes a pair of ferromagnetic layers magnetized oppositely to each other, and a nonmagnetic layer formed between the paired ferromagnetic layers. For changing the direction of magnetization of the free magnetic layer, currents are passed through the bit line and the digit line to generate two magnetic fields. The current is passed through the bit line according to timing shifted from that of current passing through the digit line, whereby turning of the magnetization of the pair of ferromagnetic layers follows the direction of the composite magnetization vector produced by the two magnetic fields, and thereby the magnetization inversion or toggling or occurs. In the toggle manner, the magnetic field alone produced by the current flowing through one of the lines cannot cause the magnetization inversion or toggling. Therefore, it is possible to prevent a malfunction that may occur when a magnetic field affects magnetization of a half-selected memory cell (see, e.g., U.S. Pat. No. 6,545,906 and “IEEE JOURNAL OF SOLID-STATE CIRCUITS”, Vol. 40, No. 1, January 2005).
A spin injection magnetization inverting method has been known as a method of inverting a magnetization direction of a ferromagnetic substance (e.g., see U.S. Pat. No. 5,695,864). In this method, a direct current is passed through a memory cell, and thereby the magnetization is inverted by an operation of spin (direction) of electrons. More specifically, a current (which will be referred to as a “spin injection current” hereinafter) is passed from one of ferromagnetic layers of a TMR element to the other ferromagnetic layer, and thereby the magnetization of the ferromagnetic layers is inverted. Since the spin injection current can be smaller in quantity than the current causing the external magnetic field, the spin injection magnetization inverting method can reduce current consumption of the MRAM as compared with the external magnetization inverting method.
As a nonvolatile magnetic memory employing this spin injection magnetization inverting method, the following nonvolatile magnetic memory has been disclosed, e.g., in Japanese Patent Laying-Open No. 2003-017782. In this memory, an insulating layer is arranged over a fixed magnetization layer, and a magnetization inverting layer having carrier-induced magnetic properties is arranged over the insulating layer. In this magnetization inverting layer, a direction of magnetization is inverted by injection of carrier spin. An electrode injecting the carrier spin into the magnetization inverting layer is arranged on the magnetization inverting layer.
As a nonvolatile magnetic memory employing the spin injection magnetization inverting method, Japanese Patent Laying-Open No. 2004-179483 has disclosed the following nonvolatile magnetic memory. In this memory, an MIS (Metal Insulator Semiconductor) junction laminated film is formed of a diode, a spin injection magnetization inversion induced layer and a tunneling magnetic resistance effect element, and bit and word lines are connected to this laminated film.
It is necessary to generate a strong magnetic field for changing the direction of magnetization of a ferromagnetic layer, and it is necessary to increase a quantity of current passed through a line for generating the strong magnetic field. Therefore, in the toggle manners disclosed in U.S. Pat. No. 6,545,906 and “IEEE JOURNAL OF SOLID-STATE CIRCUITS”, i.e., in the toggle manners requiring generation of two magnetic fields for changing the direction of magnetization, large quantities of currents flow through the bit and digit lines, which results in a problem that the current consumption for data writing is large.